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Compilers targeting the MSVC ABI have flags to select the MSVC runtime library.
Runtime library selection typically varies with build configuration because
there is a separate runtime library for Debug builds.

In CMake 3.14 and below, MSVC runtime library selection flags are added to
the default CMAKE_<LANG>_FLAGS_<CONFIG> cache entries by CMake
automatically. This allows users to edit their cache entries to adjust the
flags. However, the presence of such default flags is problematic for
projects that want to choose a different runtime library programmatically.
In particular, it requires string editing of the
CMAKE_<LANG>_FLAGS_<CONFIG> variables with knowledge of the
CMake builtin defaults so they can be replaced.

CMake 3.15 and above prefer to leave the MSVC runtime library selection flags
out of the default CMAKE_<LANG>_FLAGS_<CONFIG> values and instead
offer a first-class abstraction. The CMAKE_MSVC_RUNTIME_LIBRARY
variable and MSVC_RUNTIME_LIBRARY target property may be set to
select the MSVC runtime library. If they are not set then CMake uses the
default value MultiThreaded$<$<CONFIG:Debug>:Debug>DLL which is
equivalent to the original flags.

This policy provides compatibility with projects that have not been updated
to be aware of the abstraction. The policy setting takes effect as of the
first project() or enable_language() command that enables
a language whose compiler targets the MSVC ABI.

Note

Once the policy has taken effect at the top of a project, that choice
must be used throughout the tree. In projects that have nested projects
in subdirectories, be sure to convert everything together.

The OLD behavior for this policy is to place MSVC runtime library
flags in the default CMAKE_<LANG>_FLAGS_<CONFIG> cache
entries and ignore the CMAKE_MSVC_RUNTIME_LIBRARY abstraction.
The NEW behavior for this policy is to not place MSVC runtime
library flags in the default cache entries and use the abstraction instead.

This policy was introduced in CMake version 3.15. Use the
cmake_policy() command to set it to OLD or NEW explicitly.
Unlike many policies, CMake version 3.15.3 does not warn
when this policy is not set and simply uses OLD behavior.